Cave paintings a 20,000-year-long palimpsest?

Britain’s Natural Environment Research Council is promoting some new work on U-series dating of cave art, led by Alistair Pike of Bristol University. The most interesting result is this:

There have been surprises, though - in several caves whose art had previously been assumed to date from the same period, the new dating technique has revealed that the paintings were done in several phases, possibly over 15,000 years (25,000 years ago to just 10,000.)

The dating is done by sampling calcite crusts that form over the paintings. It’s not entirely obvious to me why dating the crust circumvents the problem they point out for radiocarbon dates of paintings:

For one thing, there's no way of telling whether these materials are contemporary with the art or much older - ancient artists could have drawn with charcoal that was already in the cave when they arrived.

Ah, well, anyway two dating methods are better than one. The Telegraph also has a story on the method, with a bit more detail about Altamira Cave:

The scientists have used their technique to date a series of famous Palaeolithic paintings in Altamira cave near Santillana del Mar, northern Spain. Known as the "Sistine Chapel of the Palaeolithic", the elaborate works were thought to date from around 14,000 years ago. But in research published today by the Natural Environment Research Council's new website Planet Earth, Dr Pike discovered some of the paintings were between 25,000 and 35,000 years old. The youngest paintings in the cave were 11,000 years old.

That’s an awfully long time.